1. Field of the Invention
This invention relates in general to spin valve magnetoresistive sensors for reading information signals from a magnetic medium and, in particular, to a spin valve sensor with multiple antiparallel coupling layers in the pinned layer for improved bias properties.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni--Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect. FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Leads 140 and 145 formed in the end regions 104 and 106, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect.
Another type of SV sensor is an antiparallel (AP)-pinned SV sensor. In AP-pinned SV sensors, the pinned layer is a laminated structure of two ferromagnetic layers separated by a non-magnetic coupling layer such that the magnetizations of the two ferromagnetic layers are strongly coupled together antiferromagnetically in an antiparallel orientation. The AP-pinned SV sensor provides improved exchange coupling of the antiferromagnetic (AFM) layer to the laminated pinned layer structure than is achieved with the pinned layer structure of the SV sensor of FIG. 1. This improved exchange coupling increases the stability of the AP-pinned SV sensor at high temperatures which allows the use of corrosion resistant and electrically insulating antiferromagnetic materials such as NiO for the AFM layer.
Referring to FIG. 2, an AP-pinned SV sensor 200 comprises a free layer 210 separated from a laminated AP-pinned layer structure 220 by a nonmagnetic, electrically-conducting spacer layer 215. The magnetization of the laminated AP-pinned layer structure 220 is fixed by an AFM layer 230. The laminated AP-pinned layer structure 220 comprises a first ferromagnetic layer 226 and a second ferromagnetic layer 222 separated by an antiparallel coupling (APC) layer 224 of nonmagnetic material (usually ruthenium (Ru)). The two ferromagnetic layers 226, 222 (FM1 and FM2) in the laminated AP-pinned layer structure 220 have their magnetization directions oriented antiparallel, as indicated by the arrows 227, 223 (arrows pointing out of and into the plane of the paper respectively).
The transfer curve (readback signal of the spin valve head versus applied signal from the magnetic disk) for a spin valve is linear and is defined by sin .theta. where .theta. is the angle between the directions of the magnetic moments of the free and pinned layers. FIG. 3a is an exemplary transfer curve for a spin valve sensor having a bias point (operating point) 300 at the midpoint of the transfer curve, at which point the positive and negative readback signals V.sub.1 and V.sub.2 (positive and negative changes in the GMR of the spin valve above and below the bias point) are equal (symmetrical) when sensing positive and negative fields having the same magnitude from the magnetic disk. FIGS. 3b and 3c illustrate transfer curves having bias points 302 and 304 shifted in the positive and negative directions, respectively, so that the readback signals V.sub.1 and V.sub.2 are asymmetrical with respect to the bias point.
The desirable symmetric bias transfer curve of FIG. 3a is obtained when the SV sensor is in its quiescent state (no magnetic signal from the disk) and the direction of the magnetic moment of the free layer is perpendicular to the magnetic moment of the pinned layer which is fixed substantially perpendicular to the disk surface. The bias point may be shifted from the midpoint of the transfer curve by various influences on the free layer which in the quiescent state can act to rotate its magnetic moment relative to the magnetic moment of the pinned layer.
The bias point is influenced by four major forces on the free layer, namely a ferromagnetic coupling field H.sub.FC between the pinned layer and the free layer, a demagnetization field H.sub.demag on the free layer from the pinned layer, a sense current field Hsc from all conductive layers of the spin valve except the free layer, and the AMR effect from the free layer which is also present in a spin valve sensor. The influence of the AMR on the bias point is the same as a magnetic influence thereon and can be defined in terms of magnitude and direction referred to herein as the AMR EFFECT. IBM's U.S. Pat. No. 5,828,529 to Gill, incorporated herein by reference, discloses an AP-pinned spin valve with bias point symmetry obtained by counterbalancing the combined influence of H.sub.FC, H.sub.demag and H.sub.SC by the influence of the AMR EFFECT on the free layer.
A problem with the prior art sensors arises as the size of spin valve sensors is decreased in order to address the need for higher storage density disk files. The AMR effect in the thinner free layer decreases and therefore the AMR EFFECT is no longer sufficient to counterbalance the influences of H.sub.FC, H.sub.demag and H.sub.SC resulting in a shift of the bias point toward a positive asymmetry. The asymmetric bias results in asymmetric readback signal response for positive and negative magnetic signals and to reduced signal output and dynamic range of the SV sensor.
Therefore there is a need for an SV sensor that provides a symmetric bias point on the transfer curve and improved signal output without sacrificing other desirable characteristics such as the strength of pinning of the pinned layer and in-file resettability of the antiferromagnetic layer.